1 Technical Field
The present invention relates to a production method for a sintered member. Specifically, the present invention relates to a technique for obtaining a sintered member with high density and high strength by forging after sintering. The sintered member has high density and high strength that are equivalent to those of a wrought steel.
2 Background Art
In a powder metallurgical method, a raw powder including a metallic powder is compacted into a green compact in a predetermined shape with predetermined dimensions. Then, the green compact is sintered at a predetermined temperature, whereby the powder particles are reliably bonded together, and a metallic product of a sintered member is manufactured. According to the powder metallurgical method, a sintered member can be formed in a shape that is near-net-shape of the product, and the sintered member is suitable for mass production. Moreover, a sintered member made of a specific material, which cannot be obtained by a wrought steel, can be produced. Therefore, the sintered members produced by the powder metallurgical method are widely used for automobile machine parts and various industrial machine parts.
In general, when a raw powder is compacted into a green compact, the green compact has spaces among the powder particles, and the spaces in the green compact remain as pores in the sintered member after sintering. Therefore, a sintered member made by the powder metallurgical method tends to have low strength compared with a member made of a wrought steel. In view of this, a great amount of alloying elements may be added to a sintered member, and the matrix of the sintered member is strengthened by the alloying elements. That is, a sintered member has been made by using an alloy that has a higher steel grade than that of an alloy used for a wrought steel. However, since the price of each alloying element has been rising recently, the cost of the raw powder has been increasing. On the other hand, by performing liquid phase sintering, a liquid phase is generated in the green compact, and the liquid phase fills up the spaces among the powder particles and thereby prevents generation of pores. In this case, a sintered member having high accuracy is difficult to obtain, and thereby machining is required after sintering. That is, a sintered member is not formed in a near-net-shape of a product, and the advantage of the powder metallurgical method is not effectively obtained.
In these circumstances, studies for improving strength of a sintered member are disclosed in, for example, PCT International Publications Nos. WO97/047418 and WO02/000378, U.S. Pat. No. 5,754,937, and Japanese Patent Application of Laid-Open No. 2004-091929. PCT International Publication No. WO97/047418 corresponds to U.S. Pat. No. 6,171,546, EP Patent Application of Laid-Open No. 0958077, Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2000-511975, and CN Patent Application of Laid-Open No. 1222105. PCT International Publication No. WO02/000378 corresponds to U.S. Patent Application of Laid-Open No. 2003/0155041, EP Patent Application of Laid-Open No. 1294511, Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2004-502028, and CN Patent Application of Laid-Open No. 1438926. U.S. Pat. No. 5,754,937 corresponds to U.S. Pat. No. 6,193,927, PCT International Publication No. WO97/043458, EP Patent Application of Laid-Open No. 0910680, and Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2001-513143. In the study, densities of a part or the entirety of a sintered member is increased by plastic deforming. Therefore, the amount of pores that may become starting points of fracture is decreased, whereby strength of a matrix is increased.
In the techniques disclosed in PCT International Publications Nos. WO97/047418 and WO02/000378, a sintered member is densified, whereby strength is partially improved. Nevertheless, the sintered member has low overall strength due to the existence of the pores. On the other hand, in the techniques disclosed in U.S. Pat. No. 5,754,937 and Japanese Patent Application of Laid-Open No. 2004-091929, a sintered member is cold forged so as to crush almost all of the pores after sintering. The sintered member has a density equivalent to that of a wrought steel, but the strength is less than that of a wrought steel.